EDM control system to maximize efficiency by controlling the gap width

Abstract

An electric discharge machine has a CPU that controls a switching element that supplies a pulse voltage to the gap between an electrode (which may be a wire electrode) and a workpiece. The CPU is able to determine the average pre-discharge time (the average time between the turning on of the switching element and the start of each electrical discharge) based on the sampling time interval and the counted number of discharges that occurred during the sampling time interval, by turning off the switching element at a set time after the start of each electrical discharge, and turning the switching element on again after a longer set time has elapsed from the start of each electrical discharge. The average gap voltage and average gap current are then calculated and displayed from this data. The feeding of the electrode and the workpiece relative to each other is controlled to reduce the difference between the counted number of discharges that occurred during the sampling time interval and a calculated desired number of discharges.

What is claimed is: 
     
       1. An electrical discharge machining system wherein an electrically conductive workpiece and an electrode are moved relative to each other with a machining gap maintained between the electrode and the workpiece, and wherein the workpiece is machined through the energy of electrical discharges at the machining gap by application of a voltage between the workpiece and the electrode, comprising: a feed device for effecting a relative movement between the workpiece and the electrode;   an electrical discharge power supply circuit for applying power from a power source to the workpiece and the electrode, said power supply circuit incorporating a switching element which closes and opens said power supply circuit;   switching control means for turning off said switching element at the end of a first predetermined time interval after a start of each electrical discharge which takes place between the workpiece and the electrode during said relative feeding movement while said switching element is held on, and for turning on said switching element at the end of a second predetermined time interval after the start of said electrical discharge, said second predetermined time interval being longer than said first predetermined time interval, a cycle time of each electrical discharge consisting of a sum of said second predetermined time interval and a pre-discharge time between the turning-on of said switching element and said start of each electrical discharge, said second predetermined time interval consisting of a sum of an electrical-discharge time represented by said first predetermined time, and a rest time following said electrical discharge time;   a counter for counting the number of actual electrical discharges which have occurred between the workpiece and the electrode during a preset sampling interval;   feed control means for controlling the operation of said feed device to regulate a rate of said relative feeding movement between the workpiece and the electrode, so as to reduce a difference between the counted number of said actual electrical discharges and a preset optimum number of electrical discharges;   arithmetic means for calculating an average predischarge time, based on said sampling interval, said second predetermined time interval and the counted number of said actual electrical discharges;   means for calculating an average gap current flowing through said machining gap, based on the calculated average predischarge time, said electrical-discharge time, said rest time, and an actual gap current flowing through said machining gap during a length of said electrical-discharge time; and   display means for indicating the calculated average gap current.   
     
     
       2. The electrical discharge machining system of claim 1, wherein said switching control means comprises: detecting means for detecting said start of each electrical discharge and generating a detection signal indicative of said start;   a first timer responsive to each said detection signal to start measurement of said first predetermined time interval;   a second timer responsive to each said detection signal to start measurement of said second predetermined time interval; and   means for turning off said switching element when said first timer has timed out, and for turning on said switching element when said second timer has timed out.   
     
     
       3. The electrical discharge machining system of claim 1, wherein said feed control means comprises means for detecting an actual feed rate of said feed device, and means for calculating a target feed rate based on said counted number of actual electrical discharges, said preset optimum number of electrical discharges, and the detected actual feed rate, said feed control means controlling the operation of said feed device so as to reduce a difference between said detected actual feed rate and the calculated target feed rate. 
     
     
       4. The electrical discharge machining system of claim 1, further comprising monitoring means for monitoring machining conditions of the machining system, said monitoring means comparing said counted number of actual electrical discharges counted by said counter to a predetermined reference number of electrical discharges which is different from said preset optimum number, said monitoring means providing a monitoring output according to a relation between said counted number and said predetermined reference number. 
     
     
       5. The electrical discharge machining system of claim 4, wherein said predetermined reference number represents the number of electrical discharges which take place during said sampling interval, provided that a time interval between the turning-on of said switching element and the start of each said electrical discharge is zero, said monitoring means providing an output signal indicative of a physical contact of said electrode with said workpiece when said counted number of actual electrical discharges is substantially equal to said predetermined reference number. 
     
     
       6. The electrical discharge machining system of claim 1, wherein said electrode consists of a metal wire which is continuously fed lengthwise along said machining gap relative to the workpiece. 
     
     
       7. The electrical discharge machining system of claim 1, further comprising means for calculating an average gap voltage applied between the workpiece and the electrode, based on said average pre-discharge time, said electrical-discharge time, said rest time, and actual gap voltages applied between the workpiece and the electrode during individual lengths of said pre-discharge and electrical-discharge times, and further comprising display means for indicating the calculated average gap voltage. 
     
     
       8. An electrical discharge machining system wherein an electrically conductive workpiece and an electrode are moved relative to each other with a machining gap maintained between the electrode and the workpiece, and wherein the workpiece is machined through the energy of electrical discharges at the machining gap by application of a voltage between the workpiece and the electrode, comprising: a feed device for effecting a relative movement between the workpiece and the electrode;   an electrical discharge power supply circuit for applying power from a power source to the workpiece and the electrode, said power supply circuit incorporating a switching element which closes and opens said power supply circuit;   switching control means for turning off said switching element at the end of a first predetermined time interval after a start of each electrical discharge which takes place between the workpiece and the electrode during said relative feeding movement while said switching element is held on, and for turning on said switching element at the end of a second predetermined time interval after the start of said electrical discharge, said second predetermined time interval being longer than said first predetermined time interval, a cycle time of each electrical discharge consisting of a sum of said second predetermined time interval and a pre-discharge time between the turning-on of said switching element and said start of each electrical discharge, said second predetermined time interval consisting of a sum of an electrical discharge time represented by said first predetermined time, and a rest time following said electrical discharge time;   a counter for counting the number of actual electrical discharges which have occurred between the workpiece and the electrode during a preset sampling interval;   feed control means for controlling the operation of said feed device to regulate a rate of said relative feeding movement between the workpiece and the electrode, so as to reduce a difference between the counted number of said actual electrical discharges and a preset optimum number of electrical discharges;   arithmetic means for calculating an average predischarge time, based on said sampling interval, said second predetermined time interval and the counted number of said actual electrical discharges;   means for calculating an average gap voltage applied between the workpiece and the electrode, based on the calculated average pre-discharge time, said electrical-discharge time, said rest time, and actual gap voltages applied between the workpiece and the electrode during individual lengths of the pre-discharge and electrical-discharge times; and   display means for indicating the calculated average gap voltage.   
     
     
       9. The electrical discharge machining system of claim 8, wherein said switching control means comprises: detecting means for detecting said start of each electrical discharge and generating a detection signal indicative of said start;   a first timer responsive to each said detection signal to start measurement of said first predetermined time interval;   a second timer responsive to each said detection signal to start measurement of said second predetermined time interval; and   means for turning off said switching element when said first timer has timed out, and for turning on said switching element when said second timer has timed out.   
     
     
       10. The electrical discharge machining system of claim 8, wherein said feed control means comprises means for detecting an actual feed rate of said feed device, and means for calculating a target feed rate based on said counted number of actual electrical discharges, said preset optimum number of electrical discharges, and the detected actual feed rate, said feed control means controlling the operation of said feed rate and the calculated target feed rate. 
     
     
       11. The electrical discharge machining system of claim 8, further comprising monitoring means for monitoring machining conditions of the machining system, said monitoring means comparing said counted number of actual electrical discharges counted by said counter to a predetermined reference number of electrical discharges which is different from said preset optimum number, said monitoring means providing a monitoring output according to a relation between said counted number and said predetermined reference number. 
     
     
       12. The electrical discharge machining system of claim 11, wherein said predetermined reference number represents the number of electrical discharges which take place during said sampling interval, provided that a time interval between the turning-on of said switching element and the start of each said electrical discharge is zero, said monitoring means providing an output signal indicative of a physical contact of said electrode with said workpiece when said counted number of actual electrical discharges is substantially equal to said predetermined reference number. 
     
     
       13. The electrical discharge machining system of claim 8, wherein said electrode consists of a metal wire which is continuously fed lengthwise along said machining gap relative to the workpiece.